The present invention relates to a process for treating brass. More particularly, the present invention is directed to a process of removing lead from the surface of brass fixtures.
Brass is an alloy composed principally of copper, tin zinc and lead. The amount of lead added to brass affects its strength, workability and machinability of the final alloy. Brass has been widely used for plumbing fittings, waterworks valves and fittings, bronze alloys and a host of other applications. Perhaps the most widespread application of brass is its use in the manufacture of faucets, valves, fittings, water meters, and related products intended for use in delivering potable water to and within commercial or residential sites. For such uses, the amount of lead that is required to affect the desired strength machinability of the brass causes concerns with the leachability of the lead into the environment where such faucets, valves, fittings and related products are used.
Lead exhibits a relatively low solubility in solid solution in brass alloys due to the atomic size of lead atoms which is larger than copper or zinc. The low solubility of lead in brass alloys causes the lead to precipitate in lead-rich regions dispersed throughout brass alloys. The tendency toward precipitation is particularly notable near the surface of brass articles. In addition to improving the machinability of brass adjacent to the surface, the precipitation of lead near the surface increases the tendency of lead present on the surface to leach into water.
Recent standards have significantly limited the acceptable amount of lead that can be leached from brass faucets, valves, fittings and related products. One such example is the Safe Drinking Water Act, amended in 1988 to limit lead in solders and fluxes to 0.2% and to limit lead in public water supply pipes and fittings to 8%.
Similarly, efforts have been underway to limit the amount of lead found in food or water. California, for example, has promulgated regulations limiting lead exposure of an individual to less than 0.5 micrograms per day. On the Federal level, the EPA, in 1991, restructured the standard of lead in drinking water from 50 parts per billion to 15 parts per billion.
While the amount of lead that can be leached from brass plumbing components is generally low, it is nonetheless possible that the amount of lead that can be leached from such components may exceed current or planned standards. It has been proposed that current standards be made even more stringent so that lead is omitted totally from brass alloys or that brass articles be treated so that minimum quantities of lead may be leached from such articles.
Various techniques which have been proposed to minimize the leaching of lead into drinking water are exemplified by United States Patent Nos. 5,453,876 to Downey, 5,601,658 to Marinas et al., 5,707,421 to Joe, 5,904,783 to Hager et al., 5,919,519 to Tallis, and 5,958,257 to Regelbrugge et al. International Patent Publication No. WO 97/06313 also exemplifies a proposed technique to minimize the leaching of lead into drinking water.
The present invention provides a simple, relatively efficient process for treating brass which removes leachable lead therefrom.
According to other features, characteristics, embodiments and alternatives of the present invention which will become apparent as the description thereof proceeds below, the present invention provides a method of treating brass articles to reduce leachable lead therein which involves:
contacting a brass article with primary treatment solution comprising an aqueous caustic solution containing a chelating agent and having a pH of about 10 to about 14.
The present invention also provides a method of treating brass articles to reduce leachable lead therein which involves contacting a brass article with a primary treatment solution which comprises an aqueous caustic solution containing about 1 to about 10 weight percent of ethylenediaminetetraacetic acid and about 1 to about 20 weight percent of sodium hydroxide and having a pH of about 10 to about 14.
The present invention further provides solution for treating brass article to reduce leachable lead therein which includes:
an aqueous caustic solution containing a chelating agent and having a pH of about 10 to about 14.
The present invention is directed to a process of removing lead from brass and bronze articles. The present invention can be used to treat a variety of brass fixtures including faucets, valves, fittings and other brass and bronze articles.
The process of the present invention involves contacting brass articles to be treated with a high pH solution containing a chelating agent. The pH of the solution should be at least about 6 to 10, and preferably above 10.
According to theory, chelating agents possess acid-base characteristics and chelation is an equilibrium reaction. Accordingly, during the course of the present invention, the inventors found that using high pH solutions in conjunction with chelating agents improved the removal of lead from brass articles.
Accordingly, an amount of caustic should be included in the treatment solution which is sufficient to raise the pH of the solution to at least about 6 to 10, and preferably above 10.
A number of chelating agents including phosphonic acids and aminopolycarboxlic acids can be used according to the present invention. Particularly suitable chelating agents include ethylenediaminetetraacetic acid (EDTA), N-hydroxyethylethylenediaminetriacetic acid (HEDTA), and diethylenetriaminepentaacetic acid (DTPA), with ethylenediaminetetraacetic acid (EDTA) being particularly suitable for purposes of the present invention.
The pH of the treatment solution can be adjusted by adding a caustic material such as an alkali metal hydroxide, an alkali metal carbonate, or an alkali metal phosphate. Alkali metal hydroxides are particularly useful, with sodium hydroxide being suitable for purposes of the present invention.
The primary treatment solution thus comprises an aqueous solution of a caustic and chelating agent. After the primary treatment bath, treated brass articles should be subjected to a treatment rinse using water. For optimal rinsing, parts may be subjected to ultrasonics at 25kHz to 40kHz, or the bath can be agitated or caused to flow or circulate.
In addition to the primary treatment solution which is also referred to herein as the primary treatment bath, an optional pretreatment solution which comprises an aqueous bath containing an organic carboxylic acid and an inorganic per-salt such as ammonium persulphate, sodium persulphate, potassium persulphate, or sodium perborate can be used according to the present invention.
A typical pretreatment bath useful for purposes of the present invention comprises an aqueous solution containing citric acid and sodium persulfate.
When a pretreatment bath is used, treated brass articles should be subjected to a pretreatment rinse using water.
In addition to the optional pretreatment bath, brass articles processed by the present invention can also be subjected to a post treatment bath.
Brass articles that are treated with a caustic such as sodium hydroxide have an appearance that may not be acceptable in the brass fixture industry. The uneven brownish-black appearance produced during treatment in the primary treatment bath is believed to be due to a copper oxide layer. Although this discoloration may not be a factor in treating brass articles whose aesthetic appearance is unimportant, the appearance of discoloration on other brass articles such as faucets can be important.
A post treatment bath containing thiourea was determined to reduce the metal oxide present on the brass parts that were darkened by the primary treatment bath. A thiourea post-treatment bath containing thiourea and ammonium chloride having a near neutral pH, e.g. about 6 was found to be particularly useful for purposes of the at present invention.